Many of components such as automobile chassis and car body structural components are conventionally produced by press-forming steel sheets having prescribed strength. In recent years, great demands have been placed on reducing the weight of automobile bodies from the viewpoint of global environment preservation. Thus, efforts are under way to increase the strength of steel sheets used and thereby reduce the sheet thickness. However, increasing the strength of steel sheets is accompanied by a decrease in press foamability of the steel sheets. As a result, difficulties are more frequently encountered in forming steel sheets into a shape of desired members.
Thus, attentions have been drawn to a technique in which steel sheets are press formed after being heated beforehand. In particular, warm press-forming is a technique in which steel sheets are press formed after being heated to a relatively low temperature. As compared with cold press-forming techniques, the warm press-forming technique allows a decrease in forming load as well as improvements in formability such as stretch flangeability and shape fixability. With these advantages, a variety of warm press-forming methods have been proposed.
For example, Patent Literature 1 proposes a method for manufacturing high-strength, press-formed products which includes heating a hot-rolled, cold-rolled or Zn-coated steel sheet containing prescribed components to a temperature of 200 to 850° C., and thereafter warm forming the steel sheet at the temperature such that a plastic strain of not less than 2% is applied to a region which requires strength so as to ensure a ratio of an increase in tensile strength exceeding 1.10.
Patent Literature 2 proposes a method for warm press-forming high-strength steel sheets in which a high-strength steel sheet is successively press formed at high speed through multistage steps. In this method, the high-strength steel sheet is rapidly heated between steps in the course of press-forming.
Further, Patent Literature 3 proposes a method for manufacturing high-strength, warm press-formed members which includes plastically deforming a high-strength steel sheet with a tensile strength of not less than 980 MPa to form a plastically deformed region with a logarithmic strain of not less than 1 in the entirety or a portion of the high-strength steel sheet, and press-forming the steel sheet at a temperature in a warm temperature range to produce a high-strength, warm press-formed member.
Meanwhile, corrosion resistance has recently become of greater interest also for components such as automobile chassis and car body structural components. In particular, there has been an increasing demand that the occurrence of blisters at flaws after coating be suppressed, namely, post-coating corrosion resistance be exhibited.